Work Done by a
Constant Force
Work is done when a force F pushes a car through a
displacement s.
Work = Force X Distance.
Work is a scalar. Unit: N.m = joule
Q1: Two men, Joel and Jerry, push against a wall. Jerry stops
after 10 min, while Joel is able to push for 5 min longer. Compare
the work they do on the wall.
a. Joel does 50% more work than Jerry.
b. Jerry does 50% more work than Joel.
c. Joel does 75% more work than Jerry.
d. Neither of them do any work.
Q2: A 102 kg man climbs a 5.0 meter high stair case at constant
speed. How much work does he do against gravity?
a. 510 J
b. 49 J
c. 5000 J
d. 2500 J
Units
System
Force
Distance
Work
SI
newton (N) meter (m) N·m = joule (J)
CGS
dyne
cm
dyn·cm = erg
BE/USC
pound (lb)
foot (ft)
foot·pound (ft·lb)
Kinetic Energy
SI Unit of Kinetic Energy: joule (J)
Kinetic energy is a scalar.
Gravitational Potential Energy
The gravitational potential energy, GPE is the energy that an
object of mass m has by virtue of its position relative to the
surface of the earth. That position is measured by the height
h of the object relative to an arbitrary zero level:
SI Unit of Gravitational Potential Energy: joule (J)
GPE is a scalar.
Energy
The Conservation of
Mechanical Energy
THE PRINCIPLE OF
CONSERVATION OF
MECHANICAL ENERGY
The total mechanical energy (E = KE + PE) of an object
remains constant as the object moves, provided that the net
work done by external nonconservative forces is zero.
Conservation of Mechanical Energy
If friction and wind resistance are ignored, a bobsled run
illustrates how kinetic and potential energy can be
interconverted, while the total mechanical energy remains
constant.
Power
The idea of power incorporates both the concepts of work
and time.
Power is work done per unit time.
Average power, P is the average rate at which work W is
done, and it is obtained by dividing W by the time t
required to perform the work:
Units
System
Force
Distance Work
Power
SI
newton
(N)
meter
(m)
N·m =
joule (J)
J/s = Watt
(W)
CGS
dyne
cm
dyn·cm =
erg
Erg/s
foot (ft)
foot·poun Ft.lb/s
d (ft·lb)
BE/USC pound
(lb)
Horsepower, hp: 1 hp = 550 ft.lb/s = 746 W
Metabolic Rates for a
young 70-kg male
Activity
Metabolic Rate (W)
Running (15 km/h)
1340
Skiing
1050
Biking
530
Walking (5 km/h)
280
Sleeping
77
Forms of Energy
So far we have considered the following forms of energy:
Kinetic energy, Gravitational potential energy, and
Mechanical energy.
Some of the other forms of energy are:
Electrical energy, Chemical energy, Nuclear energy, Thermal
energy, and Radiant energy.
Energy Transformations
Q: Give an example where gravitational potential energy is
converted into kinetic energy?
A: Falling object.
Energy Transformations
Energy Transformations in
the Human body
Part of the chemical energy stored in food is transformed into the kinetic
energy of physical activities and into the thermal energy needed to keep
our bodies at a temperature near 98.6 °F.
Energy Transformations in an
Automobile
In an automobile chemical energy of gasoline is converted into kinetic
energy, as well as electrical energy (to operate the radio, headlights, and
air conditioner), and heat (to warm the car during the winter).
Energy Transformations in a Nuclear
Power Station
The CONSERVATION OF
ENERGY
Whenever energy is transformed from one form to another, it is found
that no energy is gained or lost in the process; the total of all the
energies before the process is equal to the total of the energies after the
process. This observation leads to the conservation of energy:
Energy can neither be created nor destroyed, but can only be converted
from one form to another.
Learning how to convert energy from one form to another more
efficiently is one of the main goals of modern science and technology.